Nonlinear Optics with a Free-Electron Laser

Abstract We report on an experiment on spectroscopic infrared-visible Sum-Frequency Generation (SFG) with a Free-Electron Laser (FEL) as the IR-source. The SFG spectrum of gallium phosphide has been investigated in detail in an experiment covering the unsurpassed spectral range from 20 to 32 μm. This experiment demonstrates how the recent accessibility of FELs as truly widely tunable sources of coherent radiation can impact a well-established discipline.